1. Field of the Invention
The present invention relates to a beam irradiation device suitably used for, for example, vehicle-to-vehicle distance detectors, distance detectors, and the like.
2. Description of the Related Art
In recent years, vehicle-to-vehicle distance detectors and distance detectors using laser beams have been employed in various fields. A vehicle-to-vehicle distance detector detects the presence or absence of an obstacle and measures a distance to the obstacle by detecting reflected beams that are obtained when laser beams are radiated from a front portion of a vehicle. In this case, the laser beams are caused to longitudinally and laterally scan a target region preset in a front space. A time difference between a timing for radiating the laser beams and a timing for receiving the reflected beams is measured at each scan position, and a distance to an obstacle located in front of each scan position is calculated from a result of the measurement.
For those detectors, a so-called beam irradiation device for irradiating a target region with laser beams while performing a scan longitudinally and laterally is employed. A laser beam scan is performed by means of a scan mechanism employing a polygon mirror, a scan mechanism employing a lens actuator, or the like.
In the scan mechanism employing the polygon mirror, laser beams are radiated onto lateral faces of the polygon mirror while rotating the polygon mirror. The polygon mirror has a polygonal cross-section and mirrors are formed on the respective lateral faces thereof. By irradiating the lateral faces of the polygon mirror with laser beams while rotating the polygon mirror, the angles of incidence of the laser beams for the respective lateral faces are changed. Thus, reflected beams are scanned in the direction in which the polygon mirror is rotated.
In this scan mechanism, however, it is difficult to perform a beam scan in a direction parallel to a rotation axis of the mirror. A scan in this direction requires, for example, an additional mechanism for changing the inclination of the rotation axis of the mirror. Alternatively, the inclination angles of the respective lateral faces with respect to the rotation axis of the mirror need to be changed in advance. In this scan mechanism, furthermore, the flatness accuracy of mirror faces and the rotational state of the mirror greatly affect a beam scan state. Thus, realization of a high-accuracy scan operation requires a high-accuracy flattening work technology and also requires the use of a high-performance motor.
On the other hand, a scan mechanism employing a lens actuator is disclosed in, for example, JP 11-83988 A. In this scan mechanism, since a beam scan is performed by driving a lens, a two-dimensional scan operation can be realized with a relatively simple construction. Also, this scan mechanism does not require the use of a high-accuracy flattening work technology, a high-performance motor, or the like and thus can achieve a cost reduction in comparison with cases where a polygon mirror is used.
In this scan mechanism, however, the lens tends to be displaced undesirably due to vibrations, disturbances, or the like. As a result, there arises a problem in that the scan trajectory of laser beams deviates from a desired scan trajectory. With this scan mechanism, therefore, it may be impossible to scan a target region entirely. In this case, an inconvenience such as a failure to detect an obstacle is caused. A failure to detect an obstacle may lead to an accident when the beam irradiation device is used for a vehicle-to-vehicle distance detector or the like. Therefore, a stable scan operation must be realized in the beam irradiation device especially when the beam irradiation device is used for a vehicle-to-vehicle distance detector or the like.